Energy consumption and demand throughout the world has grown at an exponential rate. This demand is expected to continue to rise, particularly in developing countries in Asia and Latin America. At the same time, traditional sources of energy, namely fossil fuels, are being depleted at an accelerating rate and the cost of exploiting fossil fuels continues to rise. Environmental and regulatory concerns are exacerbating that problem.
Solar-related renewable energy is one alternative energy source that may provide a portion of the solution to the growing demand for energy. Solar-related renewable energy is appealing because, unlike fossil fuels, uranium, or even thermal “green” energy, there are few or no climatic risks associated with its use. In addition, solar related energy is free and vastly abundant.
Ocean Thermal Energy Conversion (“OTEC”) is a manner of producing renewable energy using solar energy stored as heat in the oceans' tropical regions. Tropical oceans and seas around the world offer a unique renewable energy resource. In many tropical areas (between approximately 20° north and 20° south latitude) the temperature of the surface sea water remains nearly constant. To depths of approximately 100 ft the average surface temperature of the sea water varies seasonally between 75° F. and 85° F. or more. In the same regions, deep ocean water (between 2500 ft and 4200 ft or more) remains a fairly constant 40° F. Thus, the tropical ocean structure offers a large warm water reservoir at the surface and a large cold water reservoir at depth, with a temperature difference between the warm and cold reservoirs of between 35° F. to 45° F. This temperature difference (ΔT) remains fairly constant throughout the day and night, with small seasonal changes.
The OTEC process uses the temperature difference between surface and deep sea tropical waters to drive a heat engine to produce electrical energy. OTEC power generation was identified in the late 1970's as a possible renewable energy source having a low to zero carbon footprint for the energy produced. An OTEC power plant, however, has a low thermodynamic efficiency compared to more traditional, high pressure, high temperature power generation plants. For example, using the average ocean surface temperatures between 80° F. and 85° F. and a constant deep water temperature of 40° F., the maximum ideal Carnot efficiency of an OTEC power plant will be 7.5 to 8%. In practical operation, the gross power efficiency of an OTEC power system has been estimated to be about half the Carnot limit, or approximately 3.5 to 4.0%. Additionally, analysis performed by leading investigators in the 1970's and 1980's, and documented in William Avery and Chih Wu, “Renewable Energy from the Ocean, a Guide to OTEC,” Oxford University Press, 1994 (incorporated herein by reference), indicates that between one quarter to one half (or more) of the gross electrical power generated by an OTEC plant operating with a ΔT of 40° F. would be required to run the water and working fluid pumps and to supply power to other auxiliary needs of the plant. On this basis, the low overall net efficiency of an OTEC power plant converting the thermal energy stored in the ocean surface waters to net electric energy has not been a commercially viable energy production option.
An additional factor resulting in further reductions in overall thermodynamic efficiency is the loss associated with providing necessary controls on the turbine for precise frequency regulation. This introduces pressure losses in the turbine cycle that limit the work that can be extracted from the warm sea water. The resulting net plant efficiency would then be between 1.5% and 2.0%
This low OTEC net efficiency compared with efficiencies typical of heat engines that operate at high temperatures and pressures has led to the widely held assumption by energy planners that OTEC power is too costly to compete with more traditional methods of power production.
Indeed, the parasitic electrical power requirements are particularly important in an OTEC power plant because of the relatively small temperature difference between the hot and cold water. To achieve maximum heat transfer between the warm sea water and the working fluid, and between the cold sea water and the working fluid large heat exchange surface areas are required, along with high fluid velocities. Increasing any one of these factors can increase the parasitic load on the OTEC plant, thereby decreasing net efficiency. An efficient heat transfer system that maximizes the energy transfer in the limited temperature differential between the sea water and the working fluid would increase the commercial viability of an OTEC power plant.
In addition to the relatively low efficiencies with seemingly inherent large parasitic loads, the operating environment of OTEC plants presents design and operating challenges that also decrease the commercial viability of such operations. As previously mentioned, the warm water needed for the OTEC heat engine is found at the surface of the ocean, to a depth of 100 ft or less. The constant source of cold water for cooling the OTEC engine is found at a depth of between 2700 ft and 4200 ft or more. Such depths are not typically found in close proximity to population centers or even land masses. An offshore power plant is required.
Whether the plant is floating or fixed to an underwater feature, a long cold water intake pipe of 2000 ft or longer is required. Moreover, because of the large volume of water required in commercially viable OTEC operations, the cold water intake pipe requires a large diameter (typically between 6 and 35 feet or more). Suspending a large diameter pipe from an offshore structure presents stability, connection and construction challenges which have previously driven OTEC costs beyond commercial viability.
Additionally, a pipe having significant length to diameter ratio that is suspended in a dynamic ocean environment can be subjected to temperature differences and varying ocean currents along the length of the pipe. Stresses from bending and vortex shedding along the pipe also present challenges. And surface influences such as wave action present further challenges with the connection between the pipe and floating platform. A cold water pipe intake system having desirable performance, connection, and construction consideration would increase the commercial viability of an OTEC power plant.
Environmental concerns associated with an OTEC plant have also been an impediment to OTEC operations. Traditional OTEC systems draw in large volumes of nutrient rich cold water from the ocean depths and discharge this water at or near the surface. Such discharge can effect, in a positive or adverse manner, the ocean environment near the OTEC plant, impacting fish stocks and reef systems that may be down current from the OTEC discharge.